1. Field of the Invention
The present invention relates generally to variable optical attenuators, and particularly to variable optical attenuator based on electrically switchable cholesteric liquid crystal reflective polarizers.
2. Description of the Prior Art
Variable optical attenuators (VOA) play a key role in current fiber optic communications. The application areas of VOAs include power level control into receivers, power control into various optical modules or sub-modules, gain-tilt control or power equalization in optical amplifier networks.
Currently there are several forms of VOAs that have been proposed. An illustration of such proposed VOAs are set forth in the following: J. M. Hartley, et.al. U.S. Pat. No. 6,253,017 (2001) disclosing a mechanically rotating VOA; C. E. Lance, et.al. U.S. Pat. No. 4,516,827 (1985) disclosing a moving optical attenuator disc; T. Iwakiri, et.al. U.S. Pat. No. 4,893,889 (1990) disclosing a VOA with an air gap between coupled fiber ends for attenuating optical power; V. R. Dhuler, et.al. U.S. Pat. No. 6,275,320 (2001) disclosing a micro-electro-mechanical system (MEMS) VOA; S. Iwatsuka, et.al. U.S. Pat. No. 5,477,376 (1995) disclosing a magneto or acoustic optical attenuator; and V. N. Morozov, et.al. U.S. Pat. No. 6,208,798 (2001) disclosing a VOA with thermo-optic attenuator and liquid crystal (LC) attenuator. Among the various forms of VOA, LC based optical attenuators have attracted much attention due to some unique features such as no moving parts, low insertion loss and low power consumption.
Current LC attenuators may be classified into two types: polarization-control and scattering. An example of polarization-controlled LC attenuators is as followed: K. Y. Wu, et.al. U.S. Pat. No. 5,963,291 (1999) disclosing a VOA with a polarization modulation with a feedback controller; R. Albert, et.al. U.S. Pat. No. 6,111,633 (2000) disclosing a polarization independent optical switch for selectively switching an optical signal; J. J. Pan, U.S. Pat. No. 5,276,747 (1994) disclosing an optical device that controls the strength of the optical signal; S. H. Rumbaugh, et.al. U.S. Pat. No. 5,015,057 (1991) disclosing a polymer-dispersed liquid crystal (PDLC) which provides attenuation control over attenuation values.
In polarization-controlled LC attenuators, unpolarized incident light is usually split by optical crystal or polarizing beam splitter into two linearly polarized beams with perpendicularly polarized directions. By transmitting through a LC cell, the polarization states of the two beams can be controlled by a voltage applied into the LC cell. Depending on the voltage level, the amount of light that can be coupled into output fiber can be adjusted. Thus optical attenuation is electrically achieved. However, polarization-control based VOAs usually require beam displacers or polarizing beam splitters to split the incident light and re-combine them, which causes alignment difficulty and high cost. Further, the attenuation bandwidths are not easily adjusted.
On the other hand, the scattering based LC attenuators utilize the light scattering effect from a so-called polymer dispersed liquid crystal (PDLC) device. Such is the occurrence in W. J. Sinclair, et.al. U.S. Pat. No. 4,364,639 (1982) which discloses a scattering liquid crystal cell whose optical transmission can be varied. In such a device, the incident light is scattered into all directions due to the index-miss-matching between the LCs and the polymer networks, when no electric field is applied. Thus, the maximum attenuation ratio can be reached. When a reasonable high voltage is applied into the device, the LC molecules are oriented align the electric direction, which causes disappearance of index-miss-matching. The light can transmit through the device with minimum insertion loss. However, PDLC scattering based VOAs cannot totally block the light due to its scattering effect. The dynamic range usually is small. Further, the attenuation bandwidths are not easily adjusted.
Shortcomings of conventional VOAs include: no reflective mode VOAs; no variability of attenuation bandwidth; high cost; and difficulty of fabrication. Therefore, a need remains in the art for reflective mode VOAs, variability of attenuation bandwidth; and VOAs that are conveniently fabricated.